A softstarter for an asynchronous three-phase motor, comprising semiconductor devices for controlling voltages applied to the motor in one, two or three of the phases by adjusting firing angles of semiconductor devices of the type turning-off at zero-crossing of the current therethrough, wherein two semiconductor devices connected in anti-parallel with each other in each phase, has been used for some time. By controlling the voltage in the respective phase a reduction of the torque and current provided to the motor is achieved during start and stop operation. The firing angle of the semiconductor devices is used for controlling the amount of energy supplied to the motor.
Each of these types of softstarters is normally provided with three such pairs of semiconductor devices, such as thyristors. However, the semiconductor devices have become a cost-determining factor of such a softstarter, so a softstarter having only two pairs of such semiconductor devices for three of the phases is used for controlling the voltage applied to three-phase motors. This means that the remaining third phase is in the form of a conductor, which cannot be switched. The present invention is related to this type of softstarters.
It is desired that a softstarter with the above-mentioned type is able to start and stop motors in a desired torque ramp in order to minimize electrical surges in the electrical supply and/or overheating in the motors and to reduce mechanical shocks or vibration which might otherwise occur under start/stop conditions causing wear and damage to the pumps, fans etc. driven by the motors. Such damage may result in higher maintenance costs and also a shortening of the lifetime on the pipe-system.
However, when the control is limited to only two phases, undesired effects in the form of, for example, a DC component in the motor current appear during starting and stopping operation. During start-up or shut-down operation a smooth and gradually increasing or decreasing torque is desired, but this DC component appearing during start-up or shut-down sometimes generates a braking or oscillating torque. This can be both disturbing and damaging to the controller system, the electric motor, and to the motor driven load.
Furthermore, it is costly to carry out measurements on the third phase that is in the form of a conductor since an extra measuring unit and connection point have to be built only for the purpose of the measurement.
A patent application, with a publication number WO 2008/033088, describes a method and a softstarter for reducing such a DC component during start and stop of a motor having three phases, only two of the three phases being controlled by two pairs of semiconductor devices, wherein firing angles of the semiconductor devices are adjusted at a zero-crossing of the current that flows through the semiconductor devices. However, this invention may still have some error in the timing when triggering the semiconductor devices, so that DC components can not be reduced efficiently.
A patent application, with a publication number WO 2008/046885, describes a control method and motor starter device, wherein a motor has three phases and each of the three phases is controlled by a pair of semiconductor devices and the motor torque is controlled in dependence on a torque error signal, based on the calculated difference between the motor torque and a reference torque value, so that the motor torque displays a rate of change with respect to time, during a stopping or starting time interval. However, this invention is related to a softstarter comprising three pairs of semiconductor devices, each pair controlling each of the three phases of the motor.